1. Field of the Invention
The present invention relates to methods of increasing the efficiency of a cyclone-fired boiler using high sodium lignite fuel, and more particularly, but not by way of limitation, to methods of modifying ash deposits produced from the combustion of high sodium lignite fuel within cyclone-fired boilers whereby such deposits can be removed using conventional techniques.
2. Description of the Prior Art
Modern utility boilers such as those utilized in power plants commonly utilize solid fuels which bring about the formation of ash deposits on heat transfer surfaces within the boilers. The problem is particularly severe in cyclone-fired boilers which utilize high sodium lignite fuel. The combustion of the fuel produces ash containing, among other things, sodium sulfate and calcium sulfate which deposits on convective heat transfer surfaces within the boiler. The deposits reduce the heat transfer from the products of combustion to the fluid being heated making the operation of the boiler less efficient, and because the deposits are not friable, periodic shutdowns of the boiler for the physical removal of the deposits are required. The term "high sodium lignite fuel" is used herein to mean solid carbonaceous fuel containing a high concentration of sodium compounds in lignite, i.e., a fuel which when combusted produces ash which when analyzed in the oxidized state contains sodium oxide (Na.sub.2 O) in an amount greater than about 4.5% by weight.
Apparatus known as soot blowers have been used heretofore for removing ash deposits from heat exchange surfaces in power boilers without shutdown of the boilers being required. However, as stated above, in cyclone-fired boilers using high sodium lignite fuel the deposits produced on the heat exchange surfaces are difficult to remove with such soot blower apparatus.
Various methods have heretofore been proposed for reducing the quantity of deposits formed or for altering the structure of the deposits to enable effective removal of the deposits using conventional soot blower apparatus. For example, the slug dosing of limestone into pulverized coal-fired burners has heretofore been utilized in attempts to modify lignite ash deposits and make them more readily removable. See Baker, B. K. and W. H. Gardiner, "Modification of Ash Behavior in Lignite Fired Boilers," presented at the Lignite Symposium held at Grand Forks, North Dakota on May 18-19, 1977, USDOE Report No. GFERC/IC-77-I. Also, slug dosing of limestone into cyclone-fired boilers has been attempted to modify the lignite ash deposits produced. However, such attempts have been unsuccessful in improving boiler efficiency and reducing down time for cyclone-fired furnaces. See Honea, F. I., Rindt, D. K., R. Middleton and D. Rothe, "The Use of Additives to Reduce Ash Fouling Problems in Lignite-Fired Boilers," presented at the Joint Power Generation Conference ASME Paper 80-JPGC/FU-3, Sept. 28-Oct. 2, 1980, Phoenix, Ariz.
U.S. Pat. No. 1,167,472 issued to Babba on Jan. 11, 1967 discloses the addition of limestone to powdered coal burned in a furnace for the purpose of modifying the properties of the ash formed in the furnace.
U.S. Pat. No. 2,905,116 issued to Sifrin et al. on Sept. 22, 1959 discloses the addition of a low quality fuel such as fly ash and a high quality fuel to a cyclone-fired furnace.
U.S. Pat. No. 3,234,580 issued to Keck on Feb. 15, 1966 discloses the periodic injection of lime to protect boiler surfaces.
By the present invention an improved method of increasing the efficiency of a cyclone-fired boiler using high sodium lignite fuel is provided whereby the ash deposits produced on heat transfer surfaces within the boiler are modified to facilitate the removal of the deposits utilizing conventional soot blower apparatus.